Love wanted to shoot in the tourist hotspot for a scene in his upcoming crime drama The Sweeney, but authorities in London turned down his request amid fears the noise of gunfire would cause chaos.
He overcame their concerns by vowing to use pioneering new 'noiseless' blank bullets utilised in training exercises by the British armed forces - and officials eventually gave permission to film after Johnson applied some bureaucratic pressure.
Love reveals Johnson is a huge fan of the original 1970s TV series on which the movie is based, and he's adamant without the London Mayor's help, the plan to shoot in Trafalgar Square would have been dropped.
He tells British magazine Live, "I always wanted this film to have a gunfight in Trafalgar Square. The authorities said flat-out no, because of the noise... But, by chance, we heard about a blank bullet that fires without sound.
"Most blank rounds used in movies create the flash for the camera and the bang for the sound recordist. But I heard from the MOD (Ministry of Defence) that they've recently started using UTM Silent Blanks, a new type of cartridge which cycles the weapon as real, explosive bullets do, but makes very little noise... The only thing we had to add in afterwards was the sound of gunfire...
"It turns out that Boris Johnson is a massive Sweeney fan. He made it happen. It's amazing how many doors open to you when you say it's The Sweeney."